Use a profile hook instead of a trace hook.
Add a cython tracer to speed up tracing.
Add Tracer.uninstall.
Clean up post-profile hook change.

Profile hooks work differently from trace hooks. Namely, it's not
possible to deactivate one for a function from the 'call' event.
Properly track coverage with tox.

Previously this was not tracing anything.
Include cython sources in the tarball.